The present invention relates to a technique and apparatus, therefore, for improving heterojunction bipolar transistor stability.
Bipolar semi-conductor devices are inherently thermally unstable, especially when biased with a low impedance bias circuit as is normally required for power amplifier applications. This is a self-feeding problem, wherein the higher temperatures of operation result in an increased current at the collector which in turn increases the temperature thereby further increasing the current. The relationship of pn junction diode current and temperature is well known, and clearly the increase in temperature results in an increase in current which in turn results in an increase in temperature. A solution to the problem of collector current run-away is to place a small but finite resistance in the emitter of the transistor. Then, as the collector current increases, the emitter voltage increases, decreasing the emitter-base voltage drop, thereby reducing the collector current back to an acceptable value. This type of negative feedback with a ballast resistor is well known in the field of bipolar junction transistors.
In most applications, the ballast resistor is optimally distributed over the emitter fingers so that the feedback can be more effectively utilized to control runaway current in an individual or small group of fingers. Often times, a thin film or implanted resistor is placed at the end of each emitter finger or pair of fingers. This technique is advantageous in that one can readily modify the resistance value by modifying the geometry of the resistor without changing the resistor process. An alternative technique is to grow or implant a lightly doped semi-conductor region above the device emitter to provide the ballasting resistance. This technique is less advantageous than the first mentioned technique because the emitter area is defined by the fabrication transistor process and often one can not change the resistor value without modifying the transistor growth and/or fabrication process. Additionally, it is often difficult to actually control the resistance of lightly doped semi-conductor materials. Accordingly, the off-finger resistance technique is often used.
While it is true that the placement of off-finger resistance will serve to reduce the current and thereby to a great extent reduce the ill-effects of joule heating, there is clearly heat generated through the operation of the device which may be detrimental to device performance. Accordingly, it is necessary to have means to dissipate the heat from the device, preferably at the emitter where the heat is generated. There are basically two methods to reduce the thermal impedance of the device. The first technique extracts the heat generated through the semi-conductor material. To optimize this structure, the material below the heat generating active area must generally be very thin on the order of microns. This low yield process hence not practical for production. An alternative method is to extract the heat generated from the top of the device through the contact metals and place these contact metals on a large heat sink. This is most readily achieved by turning the device upside-down on the heat sink. Accordingly, this process is well known in the art as flip-chip technology. In most cases, the interface between the device contacts in the heat sink is a relatively thick region of metal. This region of metal is deposited in many ways but is usually referred to as a "bump" hence flip-chip heat dissipation is often referred to as "bumping". To attach the bump to the heat sink, solders are often used and often therefore this process is known as solder bumping.
Flip-chip mounting of an HBT requires the mounting bump to be placed directly over the emitter fingers as this is where the heat is generated and therefore must be extracted. In conventional flip-chip mounting in GaAs devices, the bump would provide the path to thermal ground and the path also to the electrical ground, resulting in a low-thermal resistance device while maintaining a low ground inductance, which is important for high gain power devices. This configuration, however, does not allow one to place an off-finger ballast resistor between the emitter fingers and the ground because the ground bump must lie directly over the emitter fingers to maintain low-thermal resistance.
Turning to FIG. 1, one possible solution is shown for the extraction of heat through the heat sink as well as the necessary connection for the off-finger resistor. To this end, the mesa type structure HBT 101 having the emitter (102) base (103) and collector 104, is electrically connected to the resistor 105 off the emitter (102) by way of the metallization (106). A layer of silicon nitride or other suitable material (107) for heat dissipation is connected to the top of the metallization at the emitter as well as to the resistor (105). The metal layer (108) serves as the electrical and thermal path in this arrangement. FIG. 2 shows a top view of the schematic for the electrical contacts of the device shown in FIG. 1. This arrangement as shown in FIG. 1 runs the electrical path from the emitter fingers off to a resistor and then back through a bump which is electrically but not thermally isolated from the emitter fingers by way of the layer of heat sink material such as Si.sub.3 N.sub.4. Unfortunately, this arrangement creates a natural inductor in through the metallization from the emitter to the resistor and then back to the bump and a natural capacitor between the emitter fingers and the metallization. This extra ground inductance causes many problems including a reduced gain, and instability problems such as when the inductance resonates with the dielectric spacer capacitance to create an unstability.
Turning to FIG. 3, an alternative arrangement is shown which reduces the ill-effects of the natural inductance. The structure as shown in FIG. 3 is not practical for the following reasons. The emitter structure 301 has a well defined area which is predetermined by the area of the mask. This area is well defined as the transistor characteristices, such as frequency and power are determined by the area of the emitter mesa as is well known to one of ordinary skill in the art. Accordingly, the electrical performance of the device which is dependent upon these characteristics is also well defined by the predesignation of the emitter mesa area. The resistor 302 is fabricated through an epitaxial growth of a suitable epitaxial material, having a well defined area again defined by the mask layer. Because the area of this epitaxial material is well defined, the resistance is well defined as well. Accordingly, effecting a change in the resistance is difficult, as there are basically two techniques in which this can be carried out. The first technique would be to change the area of the resistor 302 through a change in the mask layer. However, changing the mask layer will also change the emitter mesa area and in the process would thereby change the device characteristics which are defined by the emitter mesa area as is described above. The alternative approach would be to change the epitaxial material used to change the resistance while maintaining the area constant. Unfortunately, this is also an unattractive option as a change in the epitaxial material would be prohibitively expensive. Accordingly, the device structure as shown in FIG. 3 is unattractive as it would require a significant alteration in the processing steps or a significant increase in the cost due to the differing epitaxial material used for the resistance.
Accordingly, what is needed is a device which will enable the dissipation of joule heat as well as provide the required ballast resistors without the creation of the resonator that results in the conventional technique described above. Furthermore, what is required is a technique for fabricating the HBT having the thermal dissipation characteristics as well as ballast resistors in off-finger sight which is within the standard techniques of fabrication of the HBT. Such a technique will offer both the required end product as well as the advantage of standard processing steps.